In a conventional printer used for printing computer or other data source output, a print medium, such as paper, whether in the form of cut sheets or fan fold, is moved from a paper supply past the area where the image is applied to the sheet, and into position to be picked up by the user. One widely used method for such a paper path employs a cylindrical typewriter-style platen which acts as both the prime mover for the paper and the paper support surface used during imaging. Such a system has a simple design, relatively few parts, and reliable operation. However, there are limitations in this type of system which make it inappropriate for use in a printer, such as one having an array of ink jet nozzles, for producing high resolution graphic images.
One limitation is the size of the margins on the print sheet. In a typical text printer, the leading edge of the sheet is fed around the platen until it is captured under several idler rollers which are spring loaded against the platen. The idler rollers keep the paper in contact with the platen so that when the platen rotates, the paper moves with it without slipping. The size of the print head is usually such that when the print head traverses back and forth to produce text, it travels quite close to the idler rollers. In this arrangement the head can be positioned close enough to the leading edge of the paper to produce suitable margins for a business letter. In a graphics printer, a much smaller margin is generally required. Additionally, the size of an ink jet head may be much larger than a print head used for text. When the idler rollers are positioned so that they clear this head, the resulting margin may approach two inches.
An example of a graphics printer which prints with wide end margins is a thermal printer having model number 4693D made by Tektronix, Inc. of Beaverton, OR. This printer leaves a wide trailing edge margin because an initial idler roller must be spaced from the printing zone of the platen. The leading margin is wide because the paper is captured by a clamp disposed on a large take-up drum also spaced from the printing zone. The resulting end margins are about 1.5 inches each.
Another limitation is the curved platen itself, which causes the head-to-media spacing to be different for different nozzles in the array. Head-to-media variations inherently cause ink drop placement errors and also accentuate drop placement errors caused by variations in the drop velocity. These errors make the image quality unacceptable for high resolution graphics.
Another limitation of typewriter-style systems is that certain types of ink jet printing require the paper to be heated when the ink drop is applied. While it is possible to produce a heated rotating platen, it is a difficult problem and in a relatively high speed machine the paper may not be in contact with the platen long enough to reach the required temperature.
Some typewriter-style systems allow printing to begin before the paper reaches the idler rollers by using an additional set of rollers located below the print head to advance the paper past the print head and into the main set of idler rollers. Thus at the beginning of the copy, the lower rollers drive the paper. For the majority of the copy both sets of rollers drive the paper. At the end of the copy the upper idler rollers drive the paper.
This approach poses two problems for ink jet graphics printers. First, at the beginning of the copy the paper is pushed into position in front of the head and left there in an uncontrolled state. This is not a problem for a typical dot matrix head because the head wires will naturally push the paper against the platen if required. In an ink jet machine the critical head-to-media spacing has been lost, causing drop placement errors as well as temperature variations because of loss of contact with the platen. Second, a "hand off" has been introduced when the paper passes from one set of rollers to another, causing a momentary loss of position in the amount of paper advance. The resulting variation between lines of text is not discernable to the human eye, but the same variation in a continuous graphic image is unacceptable.